An apparent act of contrition by Mel Gibson — an apologetic text message sent to his baby mama a day after he allegedly slugged her — could wind up being the most damning evidence against the “Mad Max” star in a domestic-abuse probe.

“Oksana, I wasn’t safe for you last night,” the “Braveheart” actor-director wrote Oksana Grigorieva on Jan. 7, the Web site RadarOnline reported yesterday. “I spent two hours with a therapist today and have regained some perspective.”

The seemingly repentant Gibson then appears to beg to see the Russian beauty, her son and the couple’s infant daughter.

“What I’m telling you now is that I am safe & would like to come by and make amends to you, Sascha and Lucia,” the message read. “I won’t stay, just let you say your peace and I’ll say mine. Let me know it’s alright!”

Gibson, 54, is accused of striking and choking Grigorieva, 40, injuring her teeth while she held baby Lucia. The pair are locked in a nasty custody battle over the 8-month-old girl, and Gibson’s lawyers have said he’s not guilty of abuse.

But the text message, which Radar Online said was retrieved from Oksana’s phone as part of the criminal probe, is smoking-gun evidence of his wrongdoing, experts said.

“It’s an acknowledgment that he was out of control and not in his right mind,” said Manhattan family-law attorney Daniel Clement. “If it wasn’t safe for his adult girlfriend to be with him, you can rhetorically ask, ‘How safe would it be for the young child he wants custody of?’ ”

The “Lethal Weapon” hothead allegedly sent six text messages in half an hour before Grigorieva responded.

“I have two broken front teeth and a concussion,” she wrote. “I can not c u today. I’m hurting.”

Gibson reportedly continued to grovel. “I need to see you this evening & then I promise I’ll leave you alone for as long as you want . . . Please,” he texted.

X-rays taken of Grigorieva’s injuries by a pediatrician two days after the alleged attack are further evidence in the domestic-violence investigation, TMZ reported yesterday.

Also yesterday, a report surfaced that Gibson was violent toward Grigorieva last December, one month before he’s accused of hitting her. He threw a tantrum and hurled a chair through a glass door as she and their baby cowered, according to Radar Online.

Meanwhile, sources say Grigorieva’s sister, Natalie, is suspected of leaking the tapes that capture Gibson ranting against his ex, the Hollywood Reporter said.